Fauske, H. K.

Grolmes, M. A.

UNT Digital Library

AXIAL PROPAGATION OF FREE SURFACE BOILINGINTO SUPERHEATED LIOUIDS IN VERTICAL TUBESBYM. A. Grolmes and H. K. Fauske-NOIICE-1 fits report was prepared as an account of worksponsored by the United States Government. Neitherthe United States nor the United States Atomic EnergyCommission, nor any of their employees, nor any oftheir contractors, subcontractors, or their employees,mattes any warranty, express ot implied, or assumes anylegal liability or responsibility for the accuracy, com-pleteness or usefulness of any information, apparatus,uruduct or process disclosed, or represents that its usewould not infringe privately owned rights.For Presentation At:5th International Heat Transfer Conference,Tokyo, Japan, September 2-7, 1974u r _ ; v i - ; ' ' ! j ' . , ! I 1 ' . 1 ' - . " >'•UolC'MM-USAECaARGONNE NATIONAL LABORATORY, ARGONNE, ILLINOISAXIAL PROPAGATION OF FREE SURFACE BOILING INTO SUrERlUiATED LIIJUIDS IN VERTICAL TUBESH. A. Crolmes ami 11. K. FauskeArgonnc National Laboratory, Argonne, IllinoisAbstractWASTERA unique free surface boiling phenomenon has been observed as a result of rapid depressurization of anInitially saturated or slightly subcooled stagnant liquid coluan in the absence of wall and bulk nuclea-tion sites. Closeup high-speed photographs cr water* refrigerant-11, and methyl alcohol in tubes from0.2 to 15 in. did~eter reveal chat chc initiation of violent free surface flashing (vapor plus entrainedliquid) follows from the development of Marangonl-type surface waves. The rate of propagation of theflashing surface shows evidence of choked flow limitations and proceeds at a rate which is several ordersof magnitude greater than surface evaporation (vapor only) alone. The onset of free surface flashing wasfound to be dependent upon both the degree of initial liquid superheat and the tube diameter.NOMENCLATUREAT.5-sssiPressurePressure during flashingLiquid densityLatent heat of vaporizationVapor densityInitial temperatureInitial liquid superheat defined in textSaturation temperature at reference pressureMass flow rate per unit cross section areaEntropyVapor mass fractionSubscriptso : Stagnation conditionsf : Saturated liquidEquilibrium conditionexp : ExperimentalR : ReservoirB : Base of test sectionIHTRODUCTIOHA rather unique tvo-phase flow phenomenon has beenobserved in a process of investigating rapid expul-*ion of a superheated liquid from a vertical tube.iIn these experiments, the extent of initial liquidsuperheat was achieved by rapid depressurizationof a previously saturated or slightly subcooledstagnant liquid column. In the absence of nuelea-tlon sites at the tube walls and dissolved or en-trained gas in the bulk liquid, flashing occurredonly at the free surface, i.e., no bulk flashingoccurred. After initiation, this interface fromtthlch the two-phase flow proceeded continued topropagate into the stagnant superheated liquidcolumn to the end of the same*A number of experimental studies have been reportedon the rapid depressurization of nearly saturatedliquids. However, as noted below, these studiesgenerally involved bulk nucleation and thereforediffers markedly from the present study.Terner2 reports of the rapid depressurization ofwater heated to initial temperatures of up to 238°C.However, only qualitative discussion of some resul-ting pressure-time traces were presented. Nospecial preparation of the water or test channelwas noted and no direct visual observation of theevents were reported. It is therefore difficult toassess the exact nature of the events following de-pressurization. However, the more detailed obser-vations and measurements of Edwards and O'Brien3at similar high temperatures (242°C and 2S4°C) withdepressurization to one atmosphere may likely bepertinent to the previous study. The test channelwas filled under vacuum with degassed water, pres-surized and heated to the desired temperature.Following depressurization, simultaneous pressure,temperature, and X-ray void fraction measurementswere recorded at various axial locations. Tempera-ture and pressure measurements at the sane locationIndicated thermodynamic equilibrium in the bulk ofthe fluid was attained almost instantly as theinitial rarefaction wave passed through the liquid.This was supported by X-ray void fraction evidenceof void formation within the bulk of the fluidwithin the same short time after rupture of thediagram.These X-ray observations of void formation are inagreement with the visual observations of Friz.4Friz studied the expulsion resulting from the rapiddepressurization of water at lower pressures (1-3.5 atm). A glass test section was employed andthe depressurization event was recorded photo-graphically with a high speed camera. Both de-gassed and unprepared water were employed. However,following depressurization, in both cases many bub-bles wer:e immediately observed in the bulk of thefluid below the initial liquid level. The expul-sion a'i the two-phase fluid was characterized by an"acceleration front" whose depth was reported to beseveral centimeters. On the visual evidence of theformation of many bubbles, thermodynamic equilib-rium was assumed of this acceleration front. Itwas also suggested that the overall expulsion pro-cess was a two-phase critical flow,Slailar studies with water at low pressures (1 to4 aem) were briefly reported by Le Gonidec et al.5Little detailed discussion of the phenomena was'tisiireported. However, It was indicated that pressureacasurcoie-nts indicated near saturation conditionsin the fluid follou-lnj-, dopressurination and resultssimilar to those of Rot". A wore obtained.More recently. Hooper j?t aJ..G reported on an experi-mental study of flashing of numerous liquids andproposed a new "Lav; ot Flashing." The Internalstatic pressure within the liquid during the earlypart of the transient flashing period while vaporbubbles are growing was found to assume a nearlyconstant value. This quasi-equilibriua pressure,which lies between the saturation and blowdounpressures, is given by the empirical relationshipIt is seen that Pf depends only on the initial liq-quid temperature T o and the physical properties ofthe fluid, and is independent of the initial andblovdown pressures and of the shape of the column.Equation 1 is only valid when inertia effects domi-nate and when bulk nucleation occurs.Thus, the available studies of the consequences ofrapid depressurization of various liquids wouldsuggest the immediate appearance of many bubbleswithin the bulk of the fluid. The purpose of thepresent paper is to discuss an experimental studyInvolving rapid depressurization of a number ofnearly saturated liquids in the absence of walland bulk nucleation.EXPERIMENTThe experimental facility is illustrated in Fig. 1.TIKEKl«~%j-t=L.CAUtlUL_n.""#«tSSt«E Pf1fof(«)j LIQUID• —TO VACUUM—' HESEftvOIN(A)1— S'ULLVALVe— OIAPHRASU (C tfCiARGE *MPLlfl£R%- OSCILLOSCOPECH*dCt *MI»LlFig. 1. Experimental Test ApparatusThe large reservoir (A) can be evacuated to servea* a low pressure sink. The volume of the reser-voir (% 10 ft3) is large compared to the volume ofthe tesc sections (B). Class test sections ofdiameters 2 through SO mo and lengths 10 to 80 cmwere used. One tesc section 20 mi x 30 cm lengthwa* provided with an outer jacket for the circula-tion of heating vater LO establish uniform testfluid temperature at any level above ambient up to% 80'C. After filling with the test fluid, theteit section was attached to the reservoir at thediaphragm burst assembly (C). ,\u aluminum dia-phragm separated the test iluld from the low pres-sure reservoir. The fLuid uas dupressurizetl !>yrupturing the diaphragm with a solenoid actuatedknife blade and depressurization was accomplishedin less than 2 X 10"3 sec. A short time before thediaphragm break the high-speed camera' (0) wasstarted si that the desired running speed could beestablished at the instant of depressurization.The resulting phenomena was recorded on the notionpicture iilr.i. During the expulsion, in some tests,pressures at the base of the test section (LI) andat the expansion section (E2) were measured withquartz piezoelectric pressure transducers and dis-played on a dual-be.ira memory oscilloscope (F). Theinitial fluid temperature was taken to be eitherambient temperature or the heating water c<?rapera-ture in the jacketed test section: Prior testingwith actual neasur on tints of the fluid temperatureand comparison with either ambient or heating fluidtemperature showed chis to be a sufficiently goodestimate of the test fluid temperature. Upon de-pressurization, the initial liquid superheat <!Tsiuas taken to be the initial liquid temperature Toless the saturation temperature corresponding tothe reservcir pressure T s(p r). The test fluidswere water, methyl 'ilcohol, and the freon refirger-ant 11 and 113. In many instances, with water andin some instances with methyl alcohol depressuriza-tion produced many bubbles appearing in the fluidbelow the initial liquid level (similar to previousstudies summarized<above). These bubbles appearedto come off the wall. However, with careful cleans-ing and rinsing of the test section and degassingof the water by vigorous prior boiling, this appear-ance of bubbles in the fluid below the initialliquid level after; depressurization could be com-pletely prevented.Less care was required in the case of the refriger-ants. These fluids appear to wet the glass surfacewell and do not appear to degass readily upon de-pressurization. In fact, covering the free surfaceof F-ll with glycerol permitted slow depressuriza-tion to the extent that the fluid readily sustainedliquid superheat in excess of 95°C before releasethrough nucleation at some wall cavity site. Nonethe less for all fluids mentioned above sufficientcare in test procedure would insure the initiationof free surface boiling with no bubbles in the bulkof the liquid. A more detailed description of thisphenomena follows.DESCRIPTION 01? FREE SURFACE BOILINGCloseup high-speed motion pictures of the free sur-face before, during, and after depressurizationrevealed an interesting concentric ring surfacewave pattern at the free surface being establishedwithin 1 ms of the arrival of the depressurizationfront. Thia surface wave activity was also ob-served in a separate apparatus of 6 in. diameter.In this case, the wave pattern took on a more ir-regular configuration, sometimes eel'-like, but notsustained in any simple geonetric pattern. Thesewaves are likely a result of surface tension varia-tion induced at the free surface more commonly re-ferred to .is the Marangonl effect. The onset ofauch phenomena have been discussed in terms ofstability analysis by Pearson,6 Sternling andScriven,' Vidal and Acrivos,8 and others. Vidaland Acrlvos point out the difficulty in jssessing a1biQCJ<en5reCHsoas<Li-lt3Flu66.t oom•-<.00Odo»u•uaturg.eiiItlal :Inl0 0.00.007Oo©oo•HotO*-iOB*nc preservoiiResGO82.OtO i0DouiCSJooaIuVmVg .•ssu sO. Ululiquidroir ptlal ;reser*SoM Uo•32.03* '•HsOO* 'CM*sDOCM•ft£>20§bottuas3pressatmsuredtlon,©20.CMCMCMCFVr*.enos>ion,nS"S" >uri•auauperheas3ON±100enaA'enCMoCMcno8£2CM4+9cn33*-*«MmasssuredHeaCM8>oenCMn2O\<tsr•XIICBCO01E•2culatiCalFRfOM-ll LIQUIDPCRATliHt T5*Fi I.0N -TEM ER• INITIAL LIQUID LEVEL'. I . TWO-PHASEi lOJ— • MIXTURE£ 8 i! LIQUIDAVERAGE VELCCITr106 ipsI I I ! IINTERFACEAT 04 SecPRESSURETRANSDUCER0 02 04 06 08 1.0TIME, secFig. 4. Two-phase Flashing InterfacePosition vs Timebased on energy, momentum and continuity balancewritten about either side of the interface with theassumptions of saturated liquid conditions on oneside and critical flow rate on the other. Whilethe assumption of a saturated liquid is not veri-fied by the data listed in Table 1, it is, however,Interesting to note that an equilibrium criticalflow rate evaluated at the measured base pressurePB (which is different from the exit pressure) and(2)an equilibrium quality x e g • SQ - Sf/Sfg shows goodagreement with all three fluids. However, as indi-cated in Figs. Sa and b, the flow may not be trulychoked depending u;ion the reservoir pressure con-ditions (the extent of depressurization). Note thefull expansion of the plume in Fig. 5a at thelowest reservoir pressure (characteristic of criti-cal flow) and the rather uniform ejection of theproducts at the higher reservoir pressure. Aslightly lower interface velocity Is noted in thelatter case.sI I I IF-llTWTAT9(D. Hg«2l*FINITIAL BASE PRESSUREC. .TATT,02 04 0 * 0«TIME, MCFl(. 3. Pressure Trace at Baac of Teat Sectiondutltif Two-phaie Flashing Run A,••'•• i - p z r t IFig. St. Illustration of Exit Region ofFlashing FloW Illustrating MarkedlyDifferent Flow Expansion.I - 26.5'C, PR - 0.01 atm.C°- 400 K /n2secACKNOWLEDGMENT. . , J' f—- —Fig. Sb. Illustration of Exit Region ofFlashing Flow Illustrating MarkedlyDifferent Flow Expansion.T - 26.5°C, P - 0.22 am,C°- :i50 K /m2secAn additional and perhaps equally important obser-vation is a definite threshold in the liquid super-heat required for initiating and sustaining freesurface boiling. This initial threshold superheatIs diameter-dependent for a given fluid, as shownIn Fig. 6 for Freon-11 and Methyl Alcohol. Cor thedata in Fig. 6, the initial temperature T o wasapproximately 30°C (86°F) and T s was the saturationtemperature at the reservoir pressure after rapiddepressurizition. The data should be InterpretedIn the following manner: for initial liquid super-heat less 'Jian Indicated in Fig. 6 for a given tubediameter, ;bnly evaporation takes place at the freesurface a&d there is no two-phase flashing. ForInitial liquid superheat greater than indicated fora given tube diaseter, two-phase flashing occurs;vapor plus entrained liquid are observed to proceedfrom the free surface (Fig. 5). Within the indica-ted uncertainty of the initial liquid superheat,the free surface boiling is intermittent and unsus-tained. These data along with the previous obser-vations indicate that while two-phase critical flowxatei fiay be present in this phenomenon, nass andenergy transport at the interface itself must bebetter' understood and is equally important in de-termining the actual two-phase flow regime.10 -iFREON-II •CHjOH 0TWO tnkttFUSHINOLIOWOL J T«i • • •s T* XISUSTAINEDY4CXKIIIUENT>L-ruumm -«*HOtOr IHTCIIUITTU.TrUSHWM1 • I 1 1 1 1 •10 10 MtUWTII mm.6. Test Section Dianeter Effect UponInitial Liquid Superheat Requiredto Initiate and Sustain Two-phaseFlashing fron Free SurfaceThe assistance provided by Mr. G. A. Lambert inoperating the tost facility and reducing theexperimental data is greatly acknowledged. Thiswork was performed under the auspices of the U. S.Atomic Energy Commission*III Grolmes, M. A. and H. K. Fauske, "Modeling ofSodium Expulsion with Freon-11," ASME paper70-HT-24 (May 1970).Ill Terner, E., "Shock-tube Experiments InvolvingPhase Changes," Ind. and Eng. Chem. ProcessDesign and Development, VI, No. 2, p. 85 (1962).13/ Edwards, A. R., and T. P. O'Brien, "Studies ofPhenomena Connected with the Depressurizationof Water Reactors," J. British Nuclear EnergyS o c , V9, No. 2 (April 1970).I It I Friz, G., "Coolant Ejection Studies withAnalogy Experiments," Proc. Conf. on SafetyFuels and Core Design in Large Fast PowerReactors, AXL-7120, p. 890 (October 1965).15/ Le Gonidec et a_l., "Experimental Studies onSodium Boiling," Proc. Int. Conf. on the Safetyof Fast Reactors. Aix-en-Provence, France,p. IIb-3-1 (September 1967).16/ Hooper, F. C. and N. A. Kerba, "A Law ofFlashing," Froc. Second Canadian Congress ofApplied Mechanics, Waterloo, Ontario (May 1969).171 Pearson, J. K. A., "On Convection Cells Inducedby Surface Tension," J. Fluid Mechanics, Vol.4, p. 489-500 (1958).HI Sternling, C. V. and Scriven, L. E., "Inter-facial Turbulence-hydrodynaiaic Instability andthe Marangoni Effect," AIChE Journal, Vol. 5,p. 514-523 (December 1959).191 Vidal, A. and A. Acrivos, "Effect of NonlinearTemperature Profiles on the Onset of ConvectionDriven Surface Tension Gradients," I&ECFundamentals, Vol. 7, No. 1 (1958).

Axial propagation of free surface boiling into superheated liquids in vertical tubes

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Axial propagation of free surface boiling into superheated liquids in vertical tubes

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